Many carrageenan-containing toothpastes are generally recognized as having very good properties such as acceptable physical stability, low stringiness and good rheology. Toothpastes with acceptable physical stability do not readily harden on the shelf and do not exhibit phase separation such as water or flavor separation. Low stringiness is important not only to the consumer but also in high volume manufacturing where high speed filling lines require that the toothpaste ribbon cuts off sharply from the tube. Toothpastes having good rheology will be easy to dispense from the tube yet stand up well on the brush. Moreover, these properties may be provided in a carrageenan-containing toothpaste that has appealing taste, has good cleansing effect, is easy to rinse, has excellent mouth feel, and has a smooth, pleasant appearance.
However, despite the well-known benefits of using carrageenan in toothpaste, the wider use of carrageenan has been limited by its high cost compared to other binders, especially carboxymethylcellulose (CMC). Cost is a particular issue in parts of the world where toothpaste, despite its importance for dental hygiene, remains unaffordable. Part of the higher cost of using carrageenan comes from the relatively high cost associated with obtaining the carrageenan raw material from its natural source, seaweed. Another part of the cost comes from the inherent problem of manufacturing toothpaste with binders that build viscosity. For example, heat transfer becomes less efficient as formulations become more viscous requiring greater time for heating and cooling. As formulations become more viscous they do not mix as readily and therefore require greater agitation to achieve appropriate mixing. Also, with high viscosity formulations it is more difficult to obtain consistent and accurate metering at the filling equipment. As a result of the mixing and pumping required for high viscosity formulations, most toothpastes today are prepared by a batch process. Continuous processes, which are theoretically more economical than batch processes, are generally not feasible for toothpastes with carragoenan. However, there have been attempts to design a suitable continuous process (see Catiis et al., U.S. Pat. No. 5,236,696).
Another factor to be considered when using carrageenan is the known thixotropic properties of carrageenan-containing toothpaste. This means that mechanical working of the toothpaste, for example by pumping or mixing, will reduce its viscosity. After the mechanical working is stopped, the toothpaste will regain most of its vicosity over a period of time, but it will not fully return to the viscosity level it had before the mechanical working. To compensate for this loss of viscosity, excess carrageenan must often be employed making its use less cost effective.
One approach to lowering the cost associated with carrageenan has been to seek low cost replacements for all or part of the carrageenan. When used as the sole binder in a toothpaste having a calcium-based polishing agent, carrageenan is typically present in a concentration of about 0.6% to 1.2% by weight of the toothpaste. Carrageenan can sometimes be used in lesser amounts when mixed with natural or synthetic gums and other thickeners such as CMC or xanthan. See, for example, U.S. Pat. No. 4,140,757. In cases where part of the carrageenan is replaced with other binders, oftentimes the total binder concentration must be greater than when carrageenan is used as the sole binder.
In 1982 it was first reported that desired viscosities can be obtained with less carrageenan when a composition is prepared using microwave radiation (U.S. Pat. Nos. 4,353,890; 4,457,908; 4,473,988; and 4,604,280 assigned to Colgate-Palmolive). In this process the dentifrice or cosmetic composition is treated with microwave radiation in such quantity as to raise the temperature to at least the gel-sol temperature of the carrageenan, after which the composition is allowed to quiescently cool. An advantage of this process is that improved viscosities may be obtained with somewhat less carrageenan. The preferred amount of carrageenan in toothpastes made using microwave radiation was reported to be 0.5 to 2.0% by weight, and it was further reported that viscosity increases of almost 90% may be obtained.
In these reported processes for improving the viscosity of carrageenan-containing toothpastes, a critical feature is the use of microwave radiation to raise the temperature of the formulation above the gel sol point of the carrageenan. The use of conventional heating apparatuses to raise the toothpaste temperature above the gel-sol point, as opposed to using microwave heating, has been considered an unsatisfactory means of obtaining improved viscosities (see above-noted patents). Conventional heating has been deemed unacceptable because it may cause losses of moisture and volatile flavor components, changes in flavor compositions and changes in other dentrifice consituents, due to local overheating and aeration of the paste. Additionally it has been believed that production time would be lost due to the slow heating that would be needed to avoid harm to the product.
It has now been found that viscosity enhancements of at least about 100% can be obtained when toothpaste formulations prepared from certain carrageenans are allowed to quiescently cool, for example, in the toothpaste dispenser or container. The significant viscosity enhancements may be obtained by heating the composition in a temperature range that extends well below, as well as near or above, the gel sol point of the carrageenan. The heating may be effected by either conventional or microwave heating to provide toothpastes having desirable physical properties without adversely affecting flavor and other constituents. In accordance with the methods described herein, low levels of the carrageenans, down to about 0.05% based on the weight of the toothpaste, may be used to provide a toothpaste with a desired Cuban viscosity value in the range of about 3 to 12. Compared with previous carrageenan-containing toothpastes, toothpastes of the present invention require considerably less carrageenan and are much easier to process thereby providing considerable cost savings.